Dopamine-beta-hydroxylase, an enzyme catalyzing the conversion of dopamine to norepinephrine, is found in the membranes of adrenal medullary cell chromaffin vesicles. Dopamine-beta-hydroxylase interacts with its substrate, dopamine, and calcium at the bimolecular lipid membrane to form channels through which ions pass. As the ion conductance increases linearly with the potential across the membrane, the potential itself does not open or close, i.e., "gate" the channels. To produce a conductance increase, the natural conformation of the enzyme is required, and norepinephrine, the product of the reaction catalyzed by the enzyme, cannot substitute for dopamine, its substrate. The experiments on bimolecular lipid membranes may provide a physical model for a biological action of dopamine-beta-hydroxylase. Dopamine-beta-hydroxylase in the membrane of the adrenal medullary chromaffin vesicle may combine with its substrate, dopamine, to catalyze the conversion of dopamine to norepinephrine and at the same time increase the permeability of the vesicle membrane to ions, thereby affecting synthesis, storage, or release of the vesicle catecholamines.